Since the first systematic study of strong HI absorbers, known as damped Ly-α Systems (DLAs), over 30 years ago and the realization that DLAs are the gas reservoirs that fuel the formation of galaxies, people have tried to image the galaxies associated with these absorbers. Despite valiant efforts, these studies have had limited success. The lack of detections has been generally attributed to the inherent faintness of the associated galaxy near to the bright background quasar.
The vast majority of these studies have been performed in the optical, where the quasar is bright. However, with the commissioning of the Atacama Millimeter/sub-millimeter Array (ALMA), a powerful new tool is available to search for these galaxies in the sub-millimeter. In this paper, published in Science, we have detected for the first time the cool gas traced by the fine structure line of ionized carbon, [CII], from galaxies associated with 2 DLAs at a redshift of z~4. These are the furthest galaxies to-date that have been associated with a DLA. Both the [CII] emission and the underlying dust continuum, which is also detected, suggests that these galaxies are star forming at a significant rate: 24~110 Msun/yr.
The bright [CII] emission extends about 5 kpc away from the center of the emission, marking the extent of the star-forming interstellar medium of this galaxy. The HI emission however probes the galaxy at 20 and 45 kpc, suggestive these galaxies are embedded in a large, extended HI halo, or HI disk as shown by the artist rendition here (credit A. Angelich NRAO/AUI/NSF). These observations highlight ALMA’s unique ability to detect the counterparts of DLAs, a feat that has proven elusive for all other telescopes. Here is a short video of these results, and the full paper can be found here.